ioz THE ENGINES OF THE HUMAN BODY 



the pocket valves of veins (fig. 28). The mouths of the 

 pockets are set so as to catch a back flow ; they fill 

 instantly, with the result that their sides come together 

 and block the passageway. Here again Harvey asked 

 why a channel which was supposed to allow the venous 

 nourishing blood to ebb and flow to and from the lungs 

 should be guarded by pocket or semi-lunar valves which 

 permitted a current to flow in only one direction — 

 towards the lungs. 



Harvey then told what he had succeeded in making 

 out by watching the movements of the right chambers 

 of the living heart. He confessed that at first he was 

 much puzzled ; so quick and obscure were their motions. 

 By close observation he saw that the first movement was 

 in the auricle, which became smaller and discharged its 

 blood into the ventricle ; it acted as a loading-pump. 

 Then the right ventricle drew itself together, became 

 smaller and discharged its load into the channel leading 

 to the lungs. It acted as a force-pump — one of a peculiar 

 kind. These movements followed so quickly on each 

 other that it was hard for the eye to follow and make 

 them out aright. 



Now, the physicians who listened to Harvey knew very 

 well that the lungs received two sets of vessels from the 

 heart. There were first those which came from the great 

 channel leading from the right ventricle ; they had thick 

 walls just like arteries, but as they conveyed venous 

 nourishing blood, they looked on them as veins. Then 

 there was the set of four big vessels which also conveyed 

 blood from the left auricle to the lungs ; they had thin 

 walls like veins, but as they contained arterial blood, they 

 looked upon them as the arteries of the lungs. They were 

 quite well aware when all of these vessels were followed 

 into the lungs, no matter whether they came from the 

 right side or the left, that they branched and rebranched 

 until they finally ended in vessels of the smallest size. 

 But no one had ever seen communications between the 

 two sets of terminal vessels, by which blood could pass 

 from one set to the other. Yet Harvey insisted that 



